Electrochemical techniques have been used in a broad range of sensing applications, for example for the detection and quantification of molecules of diagnostic interest in physiological samples, for sensing toxic gases and for monitoring changes in environmental parameters such as humidity.
Electrochemical impedance spectroscopy (EIS) is a technique that monitors, changes in capacitance or charge-transfer resistance associated with the changes in the local environment of a suitably modified electrode surface. Such changes can include the binding of substances (e.g. of a target species such as a biomarker) to the electrode surface as well as changes in environmental parameters such as temperature. EIS is an attractive technique for sensing applications in view, for example, of its constructional simplicity, sensitivity, selectivity and ready applicability within label-free methodologies.
In recent work it has been shown by the present inventors that electrochemical impedance methods can be applied to resolve a range of charge fluctuations within molecular films confined at electrode surfaces. These comprise changes associated with electronic dipole fluctuation and field induced ionic movement and can be resolved by Electroactive Monolayer Capacitance Spectroscopy according to their specific timescales and surface potential dependence. When these molecular films contain a moiety with orbital states that are energetically accessible (redox active) the electron transfer that results to/from the underlying metallic electrode generates a new, and sensitively potential dependent, charging process at this interface. This faradaic capacitance (known as redox capacitance, Cr) is not electrostatic and can be (for high quality molecular films with associated fast rates of heterogeneous election transfer) hundreds of times greater than the Helmholtz contribution. It has been shown that this Cr signature can be integrated into films which are additionally able to recruit specific targets of interest (such as the antigen partners of antibodies). The redox capacitance change can then be used in the establishment of a novel label free biosensing format of high sensitivity, stability and convenience. For more details, reference can be made, for example, to PCT/GB2014/051938 and to Biosensors and Bioelectronics 50 (2013)437-440.
Although these EIS techniques enable a high sensitivity, stable and convenient sensing method, the following can be noted:                (i) Redox capacitance, Cr, in effect reports (through capacitance) on the electrochemical activity of the confined redox groups at their optimal electrochemical “half wave potential”.        (ii) No information is gathered at any other potentials.        (iii) If the electrochemical activity of the film is perturbed then the measured value of Cr will change. Perturbations due to phenomena other than the specific binding/recognition event (e.g., side reactions/decomposition, changes in solvent or electrolyte penetration) would not necessarily be distinguishable from the binding/recognition event under study.        
There is thus a need for alternative, but related, sensing methods, for example based on EIS principles. Particularly attractive would be a sensing method that utilises a simple experimental set-up (e.g. a single working electrode as a probe), that is broadly applicable for the sensing of a range of different parameters, that does not suffer from one or more of the disadvantages with the known methods discussed above, and/or which has high sensitivity and/or selectivity to the chemical substance or other parameter being sensed.